1. Field of the Invention
The present invention relates to a focus servo control device for an optical system, and more specifically to a focus servo control device for an optical information reading system.
2. Description of Prior Art
Focus control is necessary in optical information reading systems in which information recorded on a recording medium is reproduced by using the reflection of a readout light beam applied on the surface of the recording medium. In order to accurately focus the readout light beam irrespective of rapid movement of the recording medium, such as a recording disc, it is general to use a focus servo loop system in which the position of an objective lens for receiving the reflection of the readout light beam is controlled in accordance with a focus error signal generated by an optical sensor means. Once the object lens is locked at an in-focus position, the position of the objective lens is automatically adjusted by the focus servo loop system so as to keep it in an in-focus state. However, at the beginning of the operation thereof, the focus servo loop system requires an initiation or a lead-in operation to search for the in-focus position. During the lead-in operation, the objective lens is, for example, moved gradually from a farthest position towards a nearest position relative to the recording surface of the recording medium. At the moment when an in-focus signal appears in the focus error signal, the focus servo loop system, which is initially set at an open loop state thereof, is immediately switched to a closed loop state in which the objective lens is automatically locked at the in focus position in accordance with the focus error signal.
In the prior art, however, the drawback is that the lead-in operation is not necessarily performed correctly in the event that a false or a spurious in-focus signal appears in the focus error signal. In other words, the focus servo loop system may be locked at a false in-focus position.